Sample inspection apparatus employing a diffraction detector

ABSTRACT

A sample inspection apparatus irradiates a sample with a conical shell of X-ray or similar radiation generating a plurality of Debye rings originating from a circular path on the sample. The apparatus is provided with two detectors. A first detector receives diffracted radiation and a second detector receives radiation which is transmitted through a coded aperture provided at a detection surface of the first detector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International ApplicationPCT/GB2018/050467 filed Feb. 22, 2018, which claims the benefit under 35USC § 119 to Great Britain Application Nos. 1703077.6, 1703078.4,1703079.2 and 1703080.0, all filed Feb. 25, 2017, the entire contents ofall of which are incorporated herein by reference.

BACKGROUND OF THE DISCLOSURE 1. Field of the Disclosure

The present disclosure relates to a sample inspection apparatusemploying a diffraction detector. The diffraction detector may be fordetection of scattered electromagnetic radiation, including X-rays.

2. Description of Related Art

It is well known to interrogate samples with X-rays. A typicalarrangement will comprise an X-ray source and a detector, with a samplebetween them. Radiation which is incident on the sample is referred toas “incoming” radiation and radiation emanating from the sample isreferred to as “outgoing” radiation.

X-ray absorption imaging is well established for producing images of theinside of objects in various public and commercial fields such assecurity scanning of luggage and other objects, and in various medicalimaging fields. Different materials attenuate X-rays to differentdegrees and these differences form the basis for imaging differentobjects.

In addition to imaging, it is also desirable to determine materials orchemicals are present in an inspected object. X-ray absorption imagingcan provide some basic material discrimination, but only in the caseswhere a material can be identified based on its absorptioncharacteristics.

As well as interacting with matter through absorption, X-rays alsointeract with matter through elastic (Rayleigh) and inelastic (Compton)scattering processes. In an elastic scattering process, an outgoingX-ray has the same wavelength as an incoming X-ray and so a diffractionpattern produced by the scattered radiation can be used to determine thelattice structure and thus material identity of the matter of the samplewhich is under inspection. This technique is commonly referred to asX-ray crystallography.

The amount of radiation that is scattered is relatively low as comparedwith the amount of radiation that is available for absorption imaging,and therefore the intensity of the X-ray radiation being measured inconventional X-ray crystallography is relatively low, requiring longintegration periods to accumulate a sufficient amount of signal foraccurate measurement. For this reason, X-ray crystallography is arelatively slow technique and is primarily used in laboratories for slowanalysis of materials, but is not generally suitable for interrogatingeveryday objects or for use in “real time” or “on-line” inspectionapplications.

Indeed, the two fields of X-ray imaging by absorption and X-raycrystallography are not normally seen as being closely related. As wellas the differences between one being much slower and confined to thelaboratory, X-ray crystallography looks at the scatter of X-rays whereasX-ray absorption looks at the primary beam, with each techniquedisregarding the portion of the radiation considered by the other.

An improved apparatus for detection of X-ray scattering is disclosed inWO 2008/149078, which is hereby incorporated by reference. Here, theheight at which a “hotspot” formed by the intersection of overlappingDebye cones is determined, and that height provides information aboutthe Bragg scattering angle of a sample under inspection.

The possibility to combine scattering and absorption data is describedin WO 2011/158047, which is hereby incorporated by reference.

A further scheme for sampling caustic rims formed by Debye rings isdescribed in WO 2014/111684, which is hereby incorporated by reference.

Even with these improvements there is still a need for furtherdevelopments in the art.

SUMMARY OF THE DISCLOSURE

According to a first aspect of the disclosure there is provided a sampleinspection apparatus comprising: a source of electromagnetic radiation;a beam former for producing a substantially conical shell of radiation,said conical shell being incident on a sample to be inspected; adetection surface arranged to receive diffracted radiation afterincidence of the conical shell beam upon the sample to be inspected; anunfocused collimator provided at or close to the detection surface andhaving a grid structure formed of cells which each stare at differentportions of the conical shell.

Optionally, the grid structure of the noise filtering collimatorcomprises a cut-out portion allowing the formation of a self-collimatinghotspot of diffracted radiation.

Optionally, the grid structure comprises lamellae which intersect toform the cells.

Optionally, the lamellae comprise a set of radial lamellae and a set ofconcentric lamellae with spaces therebetween forming a set of cells.

Optionally, additional radial lamellae are provided at one or moresuccessively more outward portions of the grid structure.

Optionally, the sample inspection apparatus comprises an occluder whichextends from a position close to or in contact with an exit side of thesample to a point which is close to the detection surface.

According to a second aspect of the disclosure there is provided asample inspection apparatus comprising: a source of electromagneticradiation; a beam forming collimator for producing a substantiallyconical shell of radiation, a first radiation detector arranged toreceive diffracted radiation after incidence of the conical shell beamupon a sample to be inspected; a coded aperture provided at a detectionsurface of the first radiation detector; and a range detector which isarranged to collect radiation transmitted through the coded aperture.

Optionally, the coded aperture is provided at a point where a radiationhotspot is expected to form.

Optionally, the coded aperture is provided at a position along asymmetry axis of the apparatus.

Optionally, the sample inspection apparatus comprises a coded aperturemask providing a plurality of apertures, and central occluder providedat one or more of the apertures of the coded aperture mask to eliminatestray illumination.

Optionally, the sample inspection apparatus comprises a system forstoring and analysing data that is collected by the first radiationdetector and/or the range detector.

Optionally, the coded aperture and the first radiation detector areintegrated.

Optionally, a body of an aperture comprises angled sidewalls that tapertowards a centre point. This aperture could be the “aperture” accordingto the main aspect, or one or more of the apertures which are providedas part an aperture mask.

According to a third aspect of the disclosure there is provided a sampleinspection apparatus comprising: a source of electromagnetic radiation;a beam former for producing a substantially conical shell of radiation,said conical shell being incident on a sample to be inspected; aradiation detection system arranged to receive diffracted radiationafter incidence of the conical shell beam upon the sample to beinspected; wherein the radiation detection system comprises amulti-planar detection surface.

Optionally, the radiation detection system comprises a plurality ofdetectors providing detection surfaces which are inclined with respectto each other.

Optionally, the plurality of detectors are in a tiled arrangement.

Optionally, an aperture is provided, being formed in a space betweenneighbouring tiled detectors.

Optionally, the sample inspection apparatus comprises a further detectorarranged to detect radiation passing through said aperture.

There can also be provided a “further detector” which is arranged todetect radiation passing through an aperture which is provided in amonolithic detection surface, formed for example of a curved detector.

According to a fourth aspect of the disclosure there is provided asample inspection apparatus comprising: a source of electromagneticradiation; a beam former for producing a plurality of coaxial andsubstantially conical shells of radiation, each conical shell having adifferent opening angle; a detection surface arranged to receivediffracted radiation after incidence of one or more of the conicalshells upon the sample to be inspected; and a set of conical shell slotcollimators provided at or close to the detection surface which eachstare at different annular regions of different corresponding conicalshells.

Optionally, the sample inspection apparatus comprises a collimator bodycomprised of a material that substantially blocks the electromagneticradiation and in which is formed a plurality of channels which aretransparent to the electromagnetic radiation and are arranged to providesaid set of conical shell slot collimators.

Optionally, a detector which provides said detection surface is anenergy-resolving detector.

Optionally, the beam former comprises a set of focussed circular orconical shell slits formed in an otherwise beam blocking material.

Optionally, the beam former comprises an optical element arranged toreceive radiation from said source of electromagnetic radiation to formconical bands of radiation.

Optionally, each conical shell of radiation originates from a differentposition along a symmetry axis of the apparatus.

Optionally, diffracted flux collection heights and/or two-theta anglesconical shell slot collimators are chosen such that each conical shellslot collimator receives diffracted flux from a sample which is producedby just one of the conical shell beams.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be described below, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 illustrates a prior art X-ray scattering detection apparatus;

FIGS. 2a, 2b, and 2c illustrate the formation of Debye cones ofdiffracted radiation formed by a collimator such as that seen in FIG. 1;

FIG. 3 illustrates another prior art X-ray scattering detectionapparatus, with a collimator structure;

FIG. 4 illustrates another prior art X-ray scattering detectionapparatus, with an alternative collimator structure;

FIG. 5 illustrates a sample inspection apparatus according to anembodiment of the disclosure, the top part showing a cross-sectionalview through a symmetry axis and the bottom part showing a plan view;

FIG. 6 shows how cells in a grid of a collimator stare at differentportions of a conical shell beam of the system as shown in FIG. 5;

FIG. 7 shows an alternative grid structure for an embodiment of acollimator;

FIG. 8 illustrates a side view cross-section of an aspect of acollimator which is provided in the apparatus of FIG. 5;

FIG. 9 illustrates a perspective view of the collimator of FIGS. 5, 6and 8;

FIG. 10 illustrates an alternative collimator which can be used with theapparatus of FIGS. 5, 6 and 8;

FIGS. 11 and 12 show aspects of how an image of an object can beobtained;

FIG. 13 shows an embodiment of an apparatus for collectingback-scattered diffracted radiation;

FIG. 14 illustrates a side view cross-section of a sample inspectionapparatus according to an embodiment of the disclosure, which providestwo detection functions; and

FIG. 15 illustrates side view cross-section of a first detector whichcan be used with the sample inspection apparatus of FIG. 14;

FIG. 16 illustrates an embodiment of a sample inspection apparatus witha set of inclined detectors;

FIG. 17 illustrates an alternative embodiment of a sample inspectionapparatus with a set of inclined detectors;

FIG. 18 illustrates an arrangement of detectors that can be used witheither of the embodiments shown in FIG. 16 or 17;

FIG. 19 illustrates a detector that can be used in various embodimentsof the disclosure;

FIG. 20 is a table that illustrates d-spacing for organic and inorganicmaterials in terms of energy and scatter angle;

FIG. 21 illustrates an embodiment of an apparatus according to thedisclosure which comprises a multi-conical shell beam system;

FIG. 22 shows primary beams generated by an extended X-ray source alongthe vertical direction or by stacked discrete x-ray sources; and

FIG. 23 illustrates further aspects of the apparatus of FIG. 21.

DETAILED DESCRIPTION OF THE DISCLOSURE

A small portion of an incoming, or “primary”, X-ray beam incident onto acrystal is scattered at measurable angles if its wavelength is similarto the lattice distances (or d-spacing) present in the crystallinematerial under inspection. For ideal, polycrystalline materialsinterrogated by pencil beams, the photon scatter follows a conedistribution, with the source of the scattering at the cone apex. These“Debye cones” form substantially circular patterns when they intersect aflat detector or other surface normally. The circles forming thesepatterns have a common centre coincident with that of the incident beamposition on the detector. The angular distribution of the scatteredintensity is unique to each different crystal structure and thus can beused to identify a material and determine characteristics such aslattice dimensions, crystallite size and percentage crystallinity. Thekey relationship between the lattice spacing (d), and the angle (θ)subtended by the diffracted or scattered radiation from a plane of atomsinside a crystal is embodied within the well-known Bragg condition:nλ=2d sinθ, in which λ is the wavelength of the incoming radiation and(n) is an integer. The angle subtended by the diffracted or scatteredradiation and the interrogating or primary radiation is 2θ (two theta).

FIG. 1 shows an example of a known apparatus 100 for gathering scatteredX-ray radiation. The apparatus 100 comprises an X-ray source 102, acollimator 104, and a detection surface 108 which includes a detector orsensor (not shown). In use, a target object 110 is interposed betweenthe X-ray source 102 and the detection surface 108. Here, a portion ofthe target object 110 is represented by the dotted lines.

The collimator 104 then produces a conical shell of X-ray radiation 112upon incidence of a cone 114 of X-rays which is produced by the X-raysource 102. A cross section of the conical shell 112 taken transverse toa primary axis 101 (z-axis) of X-ray radiation will comprise a narrowannulus of X-rays, that is, the X-rays are present in the shape of aband between a first outer cone and a second inner cone that share acommon primary axis but can have different opening angles.

The collimator 104 can comprise a ring collimator and can be made from aconventional material that might typically be used for collimatingX-rays, such as tungsten or steel. Any material can be used so long asit can significantly block the path of X-rays. In the illustratedimplementation, the collimator 104 comprises a first annulus 116 ofmaterial and a circular disc 118 of material with a diameter smallerthan an inner diameter of the solid annulus 116 and located within it,thus forming an annular aperture 120 between the disc 118 and annulus116. All three of the annulus 116, annular aperture 120 and disc 118have the same centre point. The disc 118 can be held in its positionrelative to the annulus 116 by any appropriate means such as beingattached via thin wires or by being held in place using electromagnets.It will be appreciated that any kind of arrangement is possible so longas an aperture is provided which produces a conical shell 112 of X-rayradiation. When a cone 114 of X-ray radiation is incident on thecollimator 104, X-rays pass through the aperture 120 but are blocked bythe other parts of the collimator 104, producing a conical shell 112 ofX-ray radiation.

The target object 110 is the target from which the apparatus 100 isdesigned to detect diffracted X-rays. The target object 110 can be ofnumerous forms but in the example depicted in FIG. 1 it is a plate likeobject which has a width larger in diameter than the curtain 112, butdoes not have a substantial depth.

In this embodiment detection surface 108 comprises a physical surface,but alternatively it can merely be a name given to a plane of ahypothetical surface with no solid surface present. A sensor 109 can beprovided at the detection surface 108. The sensor 109 can be located atthe centre of the surface 108 directly in line with the primary axis ofthe X-ray source.

The conical curtain 112 hits the target object 110. Since the targetobject 110 is substantially planar, the conical curtain 112 hits theobject 110 in a circular target path 122. Some of these X-rays will bescattered by the lattice of the target object 110 by Bragg diffractionand some absorbed, but much of the primary X-ray radiation willcontinue. A substantially continuous X-ray curtain 124 is then incidentupon the detection surface 108 a distance Z from the target object 110,forming an annulus of primary X-rays 126 at that surface 108.

In the embodiment described the sensor 109 is present at the centre ofthe surface 108 and has a radius sufficiently small that it is containedwithin the inner radius of annulus 126 such that none of the primaryX-ray beam is detected by the sensor 109.

Because the target object 110 contains a polycrystalline material with acertain d spacing there is X-ray diffraction causing a scatter of thephotons in a conical distribution. As mentioned above these are known as‘Debye cones’ and they are generated from every point along the circulartarget path 122 so long as the crystal structure is present. Two suchDebye cones 128, 130 are shown in FIG. 1. It is found that a small areaof concentrated radiation can be generated at a centre point 132 of thedetection surface 108 if the distance Z is set correctly. This smallarea of concentrated radiation is referred to herein as a hotspot. Thesensor 109 is arranged to be coincident with the centre point 132, orother part of the detection surface 108 where the formation of a hotspotcan be expected.

It is to be noted that Bragg's condition can be satisfied simultaneouslyby more than one wavelength, so if a broadband/polychromatic source isused together with an energy resolving sensor (to calculate wavelength),then hotspots could occur at different z positions or at a plurality ofz positions, and multiple hotspots can occur at a single position.

FIG. 2 illustrates a superposition of the cross-sections of some Debyecones from the target object 110 at the detection surface 108 for threedifferent values of the distance Z shown in FIG. 1. For ease ofillustration the footprint of the Debye cones are shown as beingcircular. This implies that the Debye cones emanate from a fixed heightfrom the detector and result from a cylindrical curtain 112 of primaryX-rays. In fact, the cross section of Debye cones will be elliptical.However, this does not change the working principle of the apparatus asdescribed herein, and circular patterns are shown for clarity ofillustration.

In all three examples the annulus of primary X-rays 126 is illustratedfor comparison purposes. In FIG. 2a , the detection surface 108 is at adistance Z=z1 where the diameters of the Debye cones are stillsignificantly smaller than the diameter of the circular target path 122.The Debye cones produce a series of circles which in practice will becontinuous but only a small selection are shown here for illustrativepurposes. At certain points 200, 202 the circles overlap thus increasingthe intensity at those points to approximately double elsewhere on thecircle and forming two rings of relatively increased intensity, referredto herein as rims. However, there are no circular paths through thecentre 132, resulting in an approximately zero intensity of X-rays atthe centre point 132, where a sensor 109 is present.

In FIG. 2b the detection surface 108 is at a distance Z=z2 where thediameters of the Debye cones are equal to the diameter of the circulartarget path 122. In FIG. 2b there are numerous overlapping points suchas points 204, 206 where two or three cones coincide increasing theintensity of X-ray radiation at those points. However, all of the conescontribute to the intensity at the very centre 132 of detection surface108 and form a hotspot where the sensor 109 is present. Accordingly, theintensity of radiation at this point is greatly increased.

In FIG. 2c , the detection surface 108 is at a greater distance Z=z3where the diameters of the Debye cones are now significantly larger thanthe diameter of the circular target path 122. In this example there areseveral points of overlap between the circles of the Debye cones such aspoints 208, 210. However, there is no point at which all of the conesare coincident. None of the circles pass through the centre point 132and therefore there is approximately zero intensity of X-rays in thecentre 132 where the sensor 109 is present.

Accordingly, there is substantially zero X-ray radiation detected at thesensor 109 at the centre point 132 in FIGS. 2a and 2c whilst there is agreat intensity from each of the cones, forming a hotspot at a singlepoint at the centre point 132 in FIG. 2 b.

Therefore, the distance Z between the target object 110 and thedetection surface 108 at which a hotspot is formed at the centre point132 acts as a measure that can help determine the identity of a materialof the target object 110. This is because the distance Z corresponds tothe scattering angle of the material and therefore is indicative of thematerial forming the target object 110. The distance can be adjusted bymoving any of the target object 110, collimator 104 or detection surface108 until the maximum radiation intensity is found.

FIG. 3 shows another example apparatus 300 for gathering scattered X-rayradiation. This apparatus shares common features with the apparatus 100of FIG. 1 and so like components are illustrated with like referencenumerals. This version is provided with a focused sensor collimator 302positioned a short distance above the detection surface 108. Thiscollimator 302 comprises a first small cone 304, a larger diameter cone306 and side portions 308. The cones 304, 306 are configured so that thesmaller one 304 is inside the larger 306, providing a channel 310between them, which has a substantially annular footprint at thedetection surface. The sensor collimator 302 is formed of material thatblocks X-rays and can suitably be formed from the same material as thesource collimator 104. It prevents X-rays hitting the sensor 109 exceptvia the channel 310.

The cones 304, 306 each have a vertical cross section which is V shapedas shown in FIG. 3. The angle of the cone side walls is chosen to focusincoming radiation at a range Z from the surface 108 along the conicalcurtain 112 which coincides with the target annulus 122. The collimator302 acts to block X-rays from hitting the sensor 109 that are notparallel with and coincident with the channel 310.

This apparatus 300 is useful where the target object 110 is not in aconvenient substantially planar form. As soon as the target object has asignificant depth there will be scatter occurring throughout its depth.Accordingly, further Debye cones will be produced going through thedepth of the target object 110 which could produce cones which fall onthe detector 109 and which can confuse the analysis. Accordingly, thesensor collimator 302 helps cut out Debye cones originating from anypoint other than distance Z. Additionally, it will help to cut out anyDebye cones caused by other materials apart from a material which isbeing tested for. If the object 110 contains the main desired materialwhich is aimed to be identified but also one or more otherpolycrystalline materials, there can be further sets of Debye cones fromthe other materials at different angles. These should also be cut out bythe sensor collimator 302.

FIG. 4 shows a further apparatus 400 for gathering scattered X-rayradiation. This apparatus shares common features with the apparatus 100of FIG. 1 and so like components are illustrated with like referencenumerals.

The apparatus 400 comprises a collimator 402 and a detector 404. Thecollimator 402 can be provided on the surface of the detector 404 or canbe provided just above it. The collimator 402 comprises a series ofcones 406 of increasing diameter and decreasing angle with channels 408between the neighbouring cones 406. The channels have a substantiallyannular footprint provided at the detection surface provided by thedetector 404. The detector 404 can comprise circular sensors 410 locatedat the bottom of each channel 408 in between each of the cones 406.

Each channel 408 is configured to collimate radiation from a givendistance along the z-axis from the detection surface 108 formed by thesensor 404, with each channel 408 being configured to collect scatteredradiation at different angles corresponding to different Debye cones.

Only Debye cones collected by the most central channel between centralcones 412 and 414 will result in a hotspot 416 with great intensity butthe annular detectors in each of the subsequent channels will stilldetect radiation from Debye cones and preferably are configured todetect a ring of Debye cone intersections. Rings of Debye coneintersections can also be referred to as caustic curves, or simplycaustics. A caustic comprises an envelope of a family of curves, formedby overlapping Debye rings, in the plane of the detector. Therefore, theshape of the caustic is a curve, which is tangent to each member of afamily of Debye rings at some point.

The apparatus 400 of FIG. 4 can be used when it is primarily intended toidentify and quantify a particular polycrystalline material within thetarget object 110.

Whilst the structures of FIGS. 3 and 4 provide some advantages, theyhave a fixed working range which is also relatively narrow, andinterrogating a relatively large inspection volume such as securityluggage screening or diagnostic imaging cannot be achieved in apractical system.

In the structures of FIGS. 3 and 4 where no collimation is provided, therange to the sample must also be known a priori. Collimation enables thedetectors to stare at a known position along the conical shell beam inorder for the diffraction angle, two-theta, to be known and d-spacingsto be calculated. The wavelength must also be known by eithermonochromising the primary beam or by using an energy resolvingdetector.

FIG. 5 shows an improved apparatus 500 for gathering scattered X-rayradiation according to an embodiment of the disclosure. This apparatusshares common features with the apparatus 100 of FIG. 1 and so likecomponents are illustrated with like reference numerals.

The apparatus 500 includes a beam former 104 for producing asubstantially conical shell of radiation. This can comprise a collimatoras mentioned above with respect to FIG. 1; but there are other types ofstructures which can produce conical shell radiation and can be referredto generally as beam formers. Example types of beam formers includemasks, of which the above annular collimator is one example, or waveguide type devices such as polycapillary optical elements which comprisea bundle of capillary optical elements arranged to form a radiation spotformed by the divergence of beams from the bundle.

Here, the apparatus 500 is provided with a collimator 502 and optionallywith an occluder 520 that is formed of suitable material to block orsubstantially attenuate X-ray radiation, such as tungsten or leadantimony alloy.

The apparatus 500 is for interrogating a sample which can be at arepresentative object range 504. The range 504 can either be known inadvance or restricted, optionally to within a given working envelope506, representing a set of values of the range over which the apparatus500 can function.

The collimator 502 comprises a grid structure which is designed toconstrain the incidence of electromagnetic radiation. The walls of thegrid are referred to herein as lamellae or septa. The lamellae compriserelatively thin plates of material, and are formed from suitablematerial to block or substantially attenuate X-ray radiation, such astungsten or lead antimony alloy. However, because they are only subjectto scattered radiation they can be relatively thin compared to thethickness of material which is required to block or attenuate primaryradiation as is required for example by the ring collimator 104. As anon-limiting example, the lamellae could have a thickness of the orderof 0.1 mm to 1 mm and a height of the order 1 cm to 30 cm.

The grid structure provides a plurality of cells 514 each of which isarranged to stare at (receive radiation from) a corresponding finitearea element of the conical shell beam 112. This is illustrated in FIG.6, which shows the cell views back-projected onto a cross-section of theprimary beam 112. This configuration maintains a nominally constanttwo-theta per cell. This unfocused arrangement is different from thearrangements of FIGS. 3 and 4 which rely on a focussing collimation.

The grid structure can comprise cells 514 of any shape. In FIGS. 5 and6, an example embodiment is shown where the lamellae can be understoodas comprising a set of radial lamellae and a set of concentric lamellae.This arrangement is desirable because the grid is designed to stare at aconical shell beam, and so each cell 514 will be well matched to aparticular given finite area element of the conical shell beam 112.

However, it will be appreciated that different forms of grid can beprovided. FIG. 7 illustrates a plan view of an alternative embodiment,in which the grid is formed from transverse and longitudinal lamellaeforming a grid of cells 714 which have a substantially squarecross-section. Each cell/element will have an angular acceptance—if weassume a deeply recessed cell then it will stare at an effectivelyunique finite area on the primary beam 112. It is to be appreciated thatcells could be formed with other shapes such as triangular or hexagonalfor example.

For the circular embodiment shown in FIGS. 5 and 6, the lamellae 508comprise a set of concentric lamellae 510 and a set of radial lamellae512. The concentric lamellae are provided at different radial positionsand are preferably centred around a centre portion 132 of the detectionsurface 108. The radial lamellae 512 are provided at different angularpositions and preferably lie along radial axes originating from a centreportion 132 of the detection surface 108.

The collimator 502 also comprises a cut-out portion 516. As seen inFIGS. 8 and 9, this cut-out portion 516 allows the formation of a focalspot of diffracted flux to form on the detection surface 108 at thecentre point 132. FIG. 8 shows a cross-sectional view of the collimator502. The side walls of the lamellae 512 slope to provide the cut-outportion 516 which at its central point exposes the surface of thedetector to a hotspot of radiation. The shape of the cut-out portion 516is designed to permit the travel of incident beams contributing to ahotspot at the central portion 132.

The structure of FIG. 3, in comparison with a structure of the presentdisclosure such as that of FIG. 5, can have better signal to noisecharacteristics due to rejection of incoherent/Compton scatterespecially for thick objects that are extended along the range axis.However, the structure of FIG. 3 only interrogates a single range (Zdistance).

The structure of the present disclosure such as that of FIG. 5, hasimproved signal to noise characteristics as compared with the structureof FIG. 4. The lamellae will also provide depth resolution (and thus twotheta assuming the wavelength is known) for a significant working range.

Referring back to FIG. 2, a selection of Debye rings are illustrated butin reality there will be a continuous series of Debye rings formedaround the annulus 126 of primary X-rays. Therefore, in FIG. 2a , afirst outer rim will be formed along a circular path as defined by aseries of overlapping points 200 and a second inner rim will be formedalong a circular path as defined by a series of overlapping points 202.In FIG. 2b radiation will focus at the hotspot 132, and an outer rimwill be formed along a circular path as defined by a series ofoverlapping points 204. In FIG. 2c , an inner rim will be formed along acircular path as defined by a series of overlapping points 210 and anouter rim will be formed along a circular path as defined by a series ofoverlapping points 214.

A rim is an envelope formed by a continuum of circles (each circle is aDebye ring) tangent to the rim. Geometrically, an envelope of a familyof curves in a plane is a curve that is tangent to each member of thefamily at some point. In optics a caustic is an envelope of rays. Forthe purposes of the present disclosure, the terms rim, envelope andcaustic can generally be used interchangeably. It is noted again that inpractice, the Debye rings will be elliptical rather than circular,having their major axes along radial directions with respect to theconical shell primary beam footprint. However, the principles of thedisclosure can be understood in a more concise manner if we make theassumption that the Debye rings are circular.

There are no other caustic curves in the region bounded by outer andinner rims (for a given Debye ring family). However, there aremultiplicity of Debye cone intersections together with continuum ofdifferent pairs of tangent Debye rings. The hotspot is a special casebecause it is formed at the intersection of a continuum of Debye ringsor equivalently a series of tangential pairs of Debye rings, where thetangent for each pair intersects at the hotspot.

In the illustration of FIG. 2, rings are shown at every radial positionand if this was the case then each rim would comprise a continuous lineof radiation of (substantially) circular shape. In practice, the angulardistribution of radiation will depend on the material e.g. preferredorientation or variable grain size, which is causing the diffraction andso the pattern of the rims can have discontinuities along the path ofthe rims. These discontinuities can form part of the distinctivesignature of a particular material and so detection of the patterns ofradiation along a rim can be useful for material identification.

This central region is in effect self-collimating. Outside of this, thelamellae 510, 512 inhibit the collection of diffracted rays which do notlie along the rims. This reduces parasitic background and so improvesthe signal to noise ratio of the collected signal. In practice thereduction of parasitic signals from different rims formed by manydifferent families (each with a constant two-theta value) of Debye coneswill improve the signal to noise ratio.

It is possible to provide a collimator 502 that employs only radiallamellae 512, without any concentric lamellae 510. However, in that casethe range to the sample needs to be known in order to calculate thetwo-theta (and hence material d-spacing) angle.

If radial lamellae 512 are combined with concentric circular lamellae510 then each cell 514 will collect diffracted flux from a correspondingregion of the primary beam 112 and will enable the calculation oftwo-theta without knowledge of the position of the sample 110 along theinterrogating beam.

The presence of both radial lamellae 512 and concentric lamellae 510defining a grid structure comprising cells 514 means that the signal tonoise ratio of the signal collected at each cell 514 is improved.

FIG. 9 shows a perspective view of a collimator structure 502, showingan example of how the radial lamellae 512, concentric lamellae 510 andcut-out portion 516 can be formed.

There can be any number of each type of lamellae, according to thedesired grid resolution, say from eight or less up to several hundred ormore.

The angles of the lamellae can be chosen to collect radiationappropriately, so that each cell 514 in the grid will stare at (receiveradiation originating from) a corresponding finite area element of theconical shell beam 112.

The collimator 502 can be manufactured by any suitable means. Thefabrication process can include steps of bonding, shearing, angling,cutting and milling, to form lamellae which comprise constant thicknessmaterials that are free from defects.

FIG. 10 shows a plan view of a collimator 800 according to analternative embodiment, in which the radial lamellae 802 have varyingpitches according to their radial position. This is advantageous becausewhen full length lamellae are employed then the finite thickness of thelamellae occlude a relatively large detector area for relatively smallradii. Also, the finite thickness limits physically the circumferentialseparation, so this can be improved with an arrangement like this.

The occluder 520 can be in the form of a cone shape and extends from aposition close to or in contact with an exit side of the sample 110 to apoint which is close to the detection surface 108. By spanning thisspace, the occluder 520 attenuates crossover diffracted flux to enablethe unambiguous calculation of two-theta angles when the location of theobject 110 under inspection is as shown in FIG. 5.

A detector 522 is provided at the detection surface 108 to collect thediffracted flux. It is possible to use various types of detectors. Apreferred embodiment would be a detector with a large sensing area thatcan detect radiation incident at each cell 514. The detector can beenergy resolving if broadband incident radiation is provided.

The detector 522 can be coupled with an appropriate system 524 thatcomprises memory and software that stores and analyses data that iscollected by the detector 522. The system 524 can comprise a computerwhich executes instructions for carrying out processing of the data. Theinstructions can be downloaded or installed from computer-readablemedium which is provided for implementing data analysis according to thedisclosure.

Material identification can be communicated to a user by a suitabledisplay or other type of indicia such as an audible of visible alarmsignal. When a computer having a display is used, graphical and audioalerts can be generated when one or more particular substances areidentified; and more complex data can be displayed in text or graphicalformat as appropriate.

An image of an object under inspection can also be generated. An exampleof the principles of this can be seen in FIGS. 11 and 12. When a beam isincident upon a flat sample of finite thickness, which is positionednormally with respect to the symmetry axis the specimen or gauge volumeis a frustum 900 of the conical shell 112. Each finite area elementcomposing the frustum corresponds to a unique cell and can be identifiedvia three variables, for example by a set of polar coordinates combinedwith a range. The gauge volume can be visualised via graphicalreconstruction in a display; rotated and moved or unfurled (net of afrustum). FIG. 11 shows an unfurled visualisation 902 in a displaywindow 904 of an electronic or digital display which can receive inputfrom a computer system 524. It will be appreciated that thevisualisation can be mapped into other shapes such as a rectangular orcircular area. FIG. 12 shows an example of how an object 1000 can bevisualised on the display 904.

The apparatus 500 of FIG. 5 has utility as a material specific probe. Itcan be termed as a “point and shoot” detection system. FIG. 5 shows astationary object but the object could be moving and the detectionsystem could analyse a swept gauge volume. One example application areawould be to have objects travelling on a conveyor belt and the system500 could be used for the detection of specified materials, such asdrugs or other contraband, in objects as they pass along the conveyorbelt. This could be done either longitudinally or transversely withrespect to the conveyor belt. That is, the X-ray source could stare downat the belt and the detector could be below it; or the X-ray sourcecould stare across the conveyor belt from one side and the detectorcould be provided across the conveyor belt at the other side.

It is also possible for a beam to track an object during a scan. Forexample, the source and beam could rotate relative to each other tosweep the beam along a linear movement direction to keep staring at anobject. This can be achieved by incorporation of image segmentation andobject tracking software provided at the computer system 524; and apre-screening system can also be provided to provided coordinates.

A pre-screener could be a; single view, multiple view or computedtomography (CT) X-ray system (or any other orthogonal screeningtechnology) that employs spectroscopic/absorption analysis e.g.dual/multi-energy analysis to provide say average atomic number andphysical density estimates within a spatial 2D/3D image. A suspectarea/volume (potential signature for contraband; drugs/explosive,homemade explosives (HME), currency etc) can be assigned a relativecoordinate position (x,y) or (x,y,z). The relative coordinate positiondetermined by the pre-screening system can then be input to thediffraction probe. The probe could then be rotated and or translated tointerrogate the region of interest. This operation could be a staticpoint and shoot mode or involve relative scan movement to acquire anintegrated signal from different relative station points along an axisthrough the threat area/volume. This combined or integrated method willhelp provide False Alarm resolution/enhanced specificity and sensitivityto reduce the probability of false alarm and increase the probability ofdetection, respectively.

In an alternative embodiment, the object 110 and the detector could betranslated relative to each other (by moving one or both of the object100 and the detector) to scan a larger area. As an example, a rasterscan can be employed. As the scan progresses, successive data frames canbe stored and analysed by the system 524. Also, a multi-emitter X-raysource could also provide the required coverage.

The device of the present disclosure can be used as a depth resolvingmaterial specific probe. Signals can be circumferentially integrated toobtain enhanced particle statistics; see dotted line 906 in FIG. 11. Theapproach includes consideration of any other axial direction or even thefull gauge volume or portions thereof.

As well as material identification, the data from the system of thepresent disclosure can also be combined with absorption data to obtainimages of objects which can be presented to a human observer; forexample as discussed in WO 2011/158047.

Various modifications of the apparatus of FIG. 5 are possible. Forexample, a detector surface could be positioned between the radiationsource and sample and be arranged to collect back-scattered radiation.One example of such a device can be understood with reference to FIG.13, which shows a primary beam 1112 emitted from a source 1102 passingthrough a detector 1108 before being incident on a sample 1110. Thesample 1110 reflects scattered radiation back towards the detector 1108and a collimator 1114 acts similarly to the collimator 502 of theprevious FIG.s to provide both angular and radial resolution by virtueof a grid structure of cells 1120 formed by radial lamellae 1124 andconcentric lamellae 1126. As with the embodiments described above, theshape of the collimator 1114 allows the formation of a hotspot at thecentral location 1132.

A device which collects back-scattered radiation can be useful for theconveyor belt example mentioned above.

It is also possible to provide at least two different detectors,arranged to detect both forward-transmitted diffracted radiation (asshown in FIG. 5) and back-scattered diffracted radiation.

According to another aspect of the disclosure, a sample inspectionapparatus comprises a source of electromagnetic radiation, a beam formerfor producing a substantially conical shell of radiation, a firstradiation detector arranged to receive diffracted radiation afterincidence of the conical shell beam upon a sample to be inspected. Thedetection surface is provided with a coded aperture at a point where aradiation “hotspot” is expected to form. This can be at a position alonga symmetry axis of the apparatus. The apparatus also comprises a rangedetector which is arranged to collect radiation transmitted through thecoded aperture.

The image from the coded aperture together with a known source anddetector configuration enables a range and a two-theta angle for asample to be calculated. Once this has been done it informs and supportsthe analysis of the diffracted flux that is collected by the firstradiation detector. That is, diffracted flux from the same Debye ringfamily as that forming a hotspot, can be identified via radialdistribution and wavelength.

The coded aperture can in one embodiment comprise a single aperture andtherefore be a pinhole camera.

The first radiation detector referred to here is similar to thatdiscussed above. It is used to detect patterns of relatively highintensity radiation (“rims”) formed by overlapping Debye rings and todetermine information that can identify the presence of a given materialby matching the detected patterns with known material signatures.

Apparatus such as that shown in FIG. 5 can be used to detect thepresence of a given material in cases where the object range is known orrestricted. This is useful in many scenarios where the position of asample to be scanned can be well controlled. However, diffracted fluxwhich is incident at the hotspot is not encoded with range informationand therefore the hotspot has only implicit material identificationinformation. Flux detected at the hotspot requires to be matched orcorrelated via radial distribution and energy or wavelength to a depthdecoded rim via surrounding collimator cells.

A sample inspection apparatus according to this aspect as shown forexample in FIG. 14 provides an additional range detection function forthe “hotspot” radiation and therefore enables sample analysis even whenthe object range is not well controlled, and thus can be used in an evenwider range of applications. This is because the hotspot alone can beused to establish material phase identification. As an example, theapparatus of FIG. 14 will cope better than the apparatus of FIG. 5 in asecurity screening application such as scanning carry-on and holdluggage, which is an example of an application where the object range isnot well controlled.

FIG. 14 illustrates an embodiment of a sample inspection apparatusaccording to an embodiment of the disclosure, which provides a firstradiation detector and a range detector. In FIG. 14, various componentsare shared with those of FIG. 5 and so like components are illustratedwith like reference numerals.

The arrangement of FIG. 14 by including a range detector providesexplicit material identification, without requiring there to be a rim,that is, the rim can fall outside the detection region of the detectoror could be occluded by clutter surrounding the sample. The rim materialidentification information is similar to the hotspot materialidentification information, but the beam paths that form the hotspot andthe rim are different. In practice (say for cluttered scenes) beam pathscan be disrupted or occluded partially or fully by other objects orsamples. It is useful to have different beam paths available foranalysis in application areas such as security screening.

Any flux which is not propagating through a point (perspective centre)will increase point-spread-function. The disclosure of this aspectenables a smaller aperture, coupled with an extended angular acceptancerange to provide enhanced determination of two-theta by virtue ofimproved image fidelity.

Here, a first detector 1400 together with a shield 1408 provide a codedaperture mask 1401. The first detector 1400 is a primary radiationdetector that functions in a manner similar to that described above forFIGS. 5 through 13. The coded aperture mask 1401 is arranged to allowradiation to pass to a second detector 1402 which performs a rangedetection function. This is achieved by the provision of an aperture1404 at the surface of the detector 1400 and a spacing 1405 at theshield 1408.

In this embodiment, the aperture 1404 is provided as the coded apertureand the spacing 1405 is a wider passage that does not form an imagingfunction as the purpose of the shield 1408 is to ensure attenuation ofthe primary beam 112 and any radiation scatter from the detector itself,so it can have a different shape such as an annular shape to providethis function. It is to be appreciated that the shield 1408 does nothave to be provided as a separate component. In an alternativeembodiment, the detector 1400 is an integral part of the coded aperturemask 1401; that is a detection surface can be formed on the surface of acoded aperture mask, or a detector can have sufficient thickness toattenuate incident radiation and be provided with an aperture so that isalso acts as a coded aperture.

This embodiment shows a single aperture 1404, which is a fundamentalimplementation of a coded aperture known as a pinhole aperture. It is tobe appreciated that an “aperture” in this context means a portion of thedetector 1400 that provides a different attenuation of incidentradiation as compared with the remaining substantial portion of thedetector surface. Although in preferred embodiments the aperture can bevoid of solid material, in alternative embodiments the aperture cancomprise solid material that lets incident radiation of the relevantenergy pass through it.

The aperture 1404 is provided at the centre point 132 of the detectionsurface 1400 and so effectively forms a lens which produces an image onthe surface of the second detector 1402. The known geometricconfiguration of the aperture 1404 and the conical shell beam 112enables the depth or range of a source of diffracted flux to becomputed. In the example illustrated, the range of the sample 110 (or acomponent part of it) to the detector surface 1400 will be determined bythe diameter of the ring 1410 of relatively intense radiation formed onthe detection surface of the second detector 1402. The relevantcalculations for determining material characteristics from the detectedradiation can be carried out by the system 524.

The second detector 1402 can comprise an annular detection surface, orcan comprise an area sensor of any other suitable shape; or any othersuitable detector. For example, one suitable camera can provide apixelated (80×80 and 2 cm square) CZT energy resolving detector e.g.energy resolution 800 eV average at 60 k eV, with a total energy range4-200 keV, with a maximum count rate of 10 million photons per second.

The ability to obtain depth information means that the effective workingrange 1406 of the system can be increased as compared with the effectiveworking range of an apparatus that does not have a range detectionfunction, compare for example the working range 506 of the apparatus ofFIG. 5 with the larger working range 1406 of the apparatus of FIG. 14.

The embodiment of FIG. 14 shows the use of a collimator 502, and theapparatus of FIG. 14 can generally be provided with any appropriate formof collimation including all the features described above with referenceto FIGS. 5 to 8. However, it is to be appreciated that the provision ofcollimation is optional. Omitting the collimator 502 achieves a lowerbill of materials, lower weight of device, and lower complexity ofmanufacture.

In alternative embodiments, other coded apertures can be provided whichare more complex as compared to a pinhole aperture as illustrated inFIG. 14. These more complex apertures comprise patterns of materialwhich are transparent to incident radiation. The modulation produced bythe specific pattern can be used to reconstruct a sharp image from acombination of images produced by each portion of the pattern. Thiscombination is achieved by a computational algorithm which can beexecuted by the system 524. The use of coded apertures would be usefulto maximise the amount of information that can be collected inconditions of low illumination levels. An improved signal to noise ratiocan be achieved.

Various modifications of the apparatus of FIG. 14 are possible. Forexample, the first and second detectors 1400, 1402 can be positionedbetween the radiation source and sample and be arranged to collectback-scattered radiation. It is also possible to provide at least twodifferent sets of first detectors and/or second detectors, arranged todetect both transmission mode (as shown in FIG. 14) and back-scattereddiffraction signals. Similar principles as illustrated in FIG. 13 wouldapply.

When designing an aperture for a coded aperture mask for use with X-rayor similar high energy radiation, it is important to ensure that theaperture has the appropriate shape but also that the surrounding mask isthick enough to attenuate or block incident radiation.

FIG. 15 shows an embodiment of a coded aperture mask 1400. Here, thebody of the detector 1400 is provided with angled sidewalls that tapertowards a centre point. This corresponds to the centre point 132 where ahotspot can be formed as described above.

FIG. 15 also shows an additional optional occluder 1500, which can beprovided also with the apparatus of FIG. 14 and other embodiments. Thecentral occluder 1500 acts to improve the signal to noise ratio byattenuating rays of radiation which are not part of the main beam ofinterest, which is focused at the centre point 132. In this embodimentthe central occluder 1500 has an upper portion 1502 and a lower portion1504 which each taper to a centre point, though other arrangements arepossible. There could be a gap at the centre point, or the occluder 1500can comprise rigidly connected or touching apexes which can provideimproved rigidity but would absorb the incident flux to some extent. Theoccluder 1500 can be held by a supporting web of struts (either hollowor solid) configured to overlap lamellae. The struts can be fabricatedfrom a material that exhibits low X-ray absorption and low X-rayscattering, and is preferably low density. A suitable choice can be alaminate plastic such as Tufnol.

According to a further aspect of the present disclosure, there isprovided a sample inspection apparatus which produces a conical shell ofradiation and which has a radiation detection system providing amulti-planar detection surface. In a preferred embodiment, a radiationdetection system comprises a plurality of detectors providing detectionsurfaces which are inclined with respect to each other. The detectorscan also be in a tiled arrangement. More preferably, each of thedetectors in the radiation detection system is arranged to stare at(collect radiation from) a given direction or portion of diffractedradiation that is scattered from a sample under inspection.

The staring angles of the detectors can be configured to accommodate anysupporting processing electronics and or cooling arrangements that wouldotherwise stop the detectors being arranged such that their detectionsurfaces tessellate a plane. The inclined detector surfaces result insemi elliptical caustics, which enable an increased two-theta coverageand increased d-spacing reach. In addition, specific points on thedetection surface can be oriented to collect diffracted flux from asample at normal incidence. The normal incident flux will exhibit areduction in spatial smearing and therefore a reduction in peakbroadening and improved peak discriminability.

Using existing radiation detectors, a radiation detection systemaccording to this aspect will comprise a plurality of detectors whichprovide planar detection surfaces and are inclined with respect to eachother. However, it will be appreciated that the disclosure can alsoprovide for curved detection surfaces which can be possible with futureadvances in flexible electronics. In this case, using curved surfacedetectors, the detection system can comprise a plurality of curvedsensors or a single curved detection surface.

In a preferred embodiment, the arrangement of the detectors or detectorof the radiation detection system provides an aperture betweenneighbouring detectors. The aperture can correspond to a centre point132 where a hotspot can be expected to form, as discussed above. Thedetectors can be provided in a tiled arrangement so that a gap betweenneighbouring detectors is provided and forms the aperture. A furtherdetector can be provided for detecting radiation passing through saidaperture.

The radiation detection system can also be provided with a collimatorwhich in a preferred embodiment could be a collimator as described abovewith reference to FIGS. 5 to 8, that is, a collimator which comprises anunfocused collimator provided at or close to a detection surface of thedetectors and having a grid structure formed of cells which each stareat different portions of the conical shell formed by the primary beam.

The apparatus according to this aspect can further comprise either apoint detector provided at the aperture or a range detector spaced fromthe aperture and provided to detect a focused image.

Various embodiments of this aspect are illustrated with respect to FIGS.16 to 19. These figures illustrate an embodiment where four detectorsare tiled and inclined with respect to each other although it will beappreciated that other numbers of detectors and other shapes ofdetectors can be used in accordance with this aspect.

Turning to FIG. 16, as before, point source 102 passes through a beamformer 104 emitting a primary beam of radiation 112 which impacts upon asample 110. A detection system is provided which, in this embodiment(and as shown also in FIG. 18), comprises four tiled detectors 1600,1602, 1604, 1606. These detectors are inclined with respect to eachother to stare in different directions illustrated by detector 1600staring in direction 1608 in FIG. 16, and detector 1602 staring indirection 1610 as illustrated in FIG. 16. In the embodiment of FIG. 16,the system is further provided with a collimator 1612. The collimator1612 can formed as single piece or can be provided by separate pieces ateach of the point detectors 1600 through 1606. The collimator 1612 canbe similar in its function to the collimator described above 502 withrespect to FIGS. 5 and following.

In addition, an apparatus according to the embodiment of FIG. 16provides point detector 1614 which is arranged to receive diffractedradiation 1616 which is focused at a hotspot. The point detector 1614 isprovided at a central portion 132 where a hotspot can be expected toform, that is, it lies along a symmetry axis of the system illustratedby z in the FIG.. The system of FIG. 16 acts in a similar way to thatdescribed above with respect to FIGS. 5 through 13, except for theinclination of the detectors 1600-1606 which provides advantages asexplained above.

FIG. 17 illustrates an alternative embodiment which acts in a similarway to the aspect described above with reference to FIGS. 14 and 15.Here, a range detector 1700 is provided that detects self-collimatedradiation passing through the coded aperture provided by the gap betweenthe detectors 1600 through 1606. As described above, the two-thetadetection function is provided by the detectors 1600-1606 and the rangedetection function is provided by the detector 1700 for improvedintelligence with material identification and imaging.

The embodiments of FIGS. 16 and 17 can also comprise a system 524similar to that described above, which comprises memory and softwarethat stores and analyses data that is collected by the detector 522.

FIG. 18 illustrates a plan view of a tiled detector arrangement that canbe used with the embodiments shown in FIG. 16 or FIG. 17. As shown here,four detectors are tiled together forming an aperture at a centre point.In addition, an embodiment of the lamellae formed by the collimator areillustrated by the grids marked on the detector surfaces. As before,these lamellae comprise a cut-out portion that allows the collection ofradiation at a hotspot.

FIG. 19 illustrates an example of the type of detector which can be usedas one of the detectors 1600 through 1606. This detector 1900 has animager 1902 mounted on one of its long sides, providing a detectionsurface parallel to the plane of that side. A plurality of thesedetectors 1900 can be provided in an arrangement where respectivedetection surfaces 1902 are inclined with respect to each other and canbe tiled together to provide an aperture. The detector tiling can have agap or gaps and or inclinations to accommodate an intermediary codedaperture. It is to be appreciated that this detector 1900 is shown as anillustrative example only.

The aperture provided between tiled detectors according to this aspectof the disclosure can be provided by simply placing the detectorscarefully together in a given arrangement to form a small gap betweentheir respective edges. Alternatively, the existing housings of one ormore of the detectors can be profiled or cut out in order to form anaperture.

In alternative embodiments, a separate bracket could be provided wherethe bracket is arranged to hold the detectors in position and is shapedto provide an aperture. The shape of the aperture of such a bracket canresemble the shape of the aperture shown in FIG. 15, for example. It canalso be provided with an occluder for blocking out stray radiation,again in a manner similar to that shown in FIG. 15.

According to a further aspect of the disclosure, there is provided anapparatus for detection of scattered radiation which comprises amulti-conical shell beam system. This enables analysis of a plurality ofdiffraction (two-theta) angles for material analysis.

FIG. 20 illustrates a table of d-spacing for organic and inorganicmaterials in terms of energy and scatter angle. According to the key,materials 2000 are organic, assumed to have d-spacings of between 2 and0.225 nm, and materials 2002 are inorganic, assumed to have d-spacingsof between 0.4 and 0.115 nm. The energy in keV is plotted vertically andthe bars show successive two-theta angles for each type of material,from two degree to 16 degrees.

The table is populated according to the following equation:

$E = {\frac{6.2}{d\; \sin \; \theta}{keV}}$

This equation is derived by combining Bragg's condition with thePlanck-Einstein relationship.

It can be appreciated from the FIG. 20 that for a relatively large rangeof d-spacings to be calculated via a single diffraction reading or scanthen a degree of compromise is required. For example, in a securityscreening scenario the explosive substances TNT and HMX have manysimilar diffraction intensity peaks across the d-spacing range 0.8 to0.2 nm thus a multiple two-theta angle arrangement would be anadvantage. In contrast, PETN and sodium stearate have a single dominantdiffraction maxima at 0.38 and 0.4 nm respectively and detection from asingle, fixed angle arrangement would be more appropriate.

In the case of drug targets, a similar analysis can be performed e.g.cocaine hydrochloride produces diffraction maxima over an extendedenergy (d-spacing) range whereas amphetamine sulphates can be identifiedfrom a single peak at approximately 1.5 nm.

Other considerations include the variable thickness of potential threatmaterials/samples and or the presence of cluttering objects, which areencountered routinely in security screening applications e.g. baggage ormobile electronic device screening. Thus, relatively smaller two-thetaangles and higher energy interrogating photons can be employed toovercome the effects of sample absorption and or absorbing clutter toprovide rapid detection and identification. In such applications theinevitable effect of some geometrical broadening (cot(theta) effect)associated with higher energy diffraction peaks can be a secondaryconsideration, especially in low exposure “real-time” screening tasks.Thus in practice some lower fidelity signal is preferable to aninsufficient amount of a “potentially” higher fidelity signal.

According to this aspect of the disclosure, the apparatus is providedwith a beam former for producing a set of concentric, conical shells ofradiation from an X-ray source. For example, the beam former can employfocussed circular or conical shell slits formed in an otherwise beamblocking material. Alternatively, X-rays from an isotropic source canredirected by an optical element or device to form conical bands ofX-rays. In addition, each conical shell of X-rays can originate from adifferent position along the symmetry axis (z-axis) of the system. Thiscan be understood with reference to FIG. 22, which shows primary beamsgenerated by an extended X-ray source along the vertical (z-axis)direction or by stacked discrete x-ray sources. Each source has acorresponding beam shaper and can support a constant opening beam angle(FIG. 22a ) or different beam angles (FIG. 22b ). The primary beams ofFIG. 22 could replace the primary beams of FIG. 21, with othercomponents of FIG. 21 being similar.

The conical shell beams can be incident upon a sample in an inspectionvolume and diffracted radiation is collected by a set of conical shellslot collimators which each stare at different annular regions on aconical shell of primary rays.

This resultant configuration can be considered as a series of concentricbut independent diffraction systems, that is, each conical shell and itscorresponding collimator form a discrete system as there is nominally nodiffracted flux coupling between each system. A maximum working rangedepends on the angular separation of each concentric conical shelltogether with the staring angle (two theta) of each conical shell slotcollimator. The values of these parameters should be chosen so that if asample is presented within an inspection volume within the resultingworking range then each conical shell slot collimator will stare at justone conical shell.

An example of such an arrangement is shown in FIG. 21. Here, a beamformer 2110 modifies radiation from a radiation source 2112 to produce aplurality of conical shell beams 2100. These shell beams are incidentupon a work surface 2102 and are incident upon an occluder 2104 whichallows corresponding diffracted flux from a sample can be incident upona diffraction detector 2106. The occluder 2104 has a body formed from amaterial suitable for blocking incident radiation, such as tungstenwhich can be suitable for blocking x-ray radiation, and has formedwithin it a plurality of channels 2108 for allowing diffracted radiationto pass through to the diffraction detector 2106. The working surface2102 is transparent to the diffracted radiation. Each of the conicalshell slot collimators stare at different annular regions on differentconical shells of primary radiation. This is illustrated at referencenumeral 2110 and understood with reference to both parts (a) and (b) ofFIG. 21.

Each conical shell slot collimator (CSSC) stares at an annular region ona conical shell of primary x-rays. An annular (or partially annular)specimen path is formed when a sample occupies this region. Thediffracted photons from the sample travel along a known trajectoryspecified by the collimator two-theta angles. This information incombination with the energy/wavelength (Planck-Einstein relation)quantification via an energy resolving detector enables d-spacings for asample to be calculated from Bragg's condition.

As mentioned above, each collimator should receive diffracted photonsonly from the correct or corresponding incident beam to enable theunequivocal calculation of d-spacings and other structural parameters,for example preferred orientation and grain size. If this condition isnot satisfied then two or more different two-theta angles can beassociated with the photons received by a collimator, which will reduceboth the sensitivity and specificity and potentially destroy theanalytical capability of the technique.

To ensure that diffracted flux from a sample can be received only at aknown two-theta angle then the following design criteria and analysishas to be adopted.

The potential for the collection of parasitic diffracted flux can beappreciated from FIG. 22, which illustrates a multi-conical shell beamsystem including consideration of the outermost two beams n and n-1. Asample at beam n produces diffracted flux received by its correspondingcollimator at diffraction angle 2θ_(n). However, a sample closer to thesource along beam n can produce parasitic flux at diffraction angle2θp_(n,n-1) that can be received by the diffraction collimator for beamn-1, as per the following equation

2θp _(n,n-1)=2θ_(n-1)+φ_(n)-φ_(n-1)   (1)

Where the angular separation between the two conical shell interrogatingbeams is φ_(n)-φ_(n-1).

It is critical that the angular distribution of the conical shell beams,diffracted flux collimators and associated diffracted flux collectionheights be configured to avoid the collection parasitic rays. Forexample, the height Hp_(n,n-1) at which parasitic diffracted flux can becollected from a sample irradiated by beam n by the collimator aimed atbeam n-1 is given by

$\begin{matrix}{{Hp}_{n,{n - 1}} = \frac{{Op}_{n,{n - 1}}\cos \; \phi_{n}{\sin \left\lbrack {\left( \frac{\pi}{2} \right) - {2\theta \; p_{n,{n - 1}}} + \phi_{n}} \right\rbrack}}{\sin \; 2\; \theta \; p_{n,{n - 1}}}} & (2)\end{matrix}$

where

0p _(n,n-1) =S _(n,n-1)+0_(n-1)   (3).

The separation of the two conical shell beams on the face of thecollimator S_(n,n-1) can be calculated from simple geometricconsiderations involving the chosen conical shell beam half openingangles φ and the point source height L. The collimator offset distance0_(n-1) from the corresponding shell beam is given by

$\begin{matrix}{O_{n - 1} = \frac{H_{n - 1}\sin \; 2\; {\theta \;}_{n - 1}}{\cos \; \phi_{n - 1}{\sin \left\lbrack {\left( {\pi/2} \right) - {2\theta_{n - 1}} + \phi_{n - 1}} \right\rbrack}}} & (4)\end{matrix}$

the chosen height to collect diffracted flux is H_(n-1).

To ensure that parasitic diffracted photons from a sample at beam ncannot be collected by the collimator at beam n-1 the followingcondition has to implemented

Hp_(n,n-1)>H_(n)   (5)

and similarly for all combinations of two or more beams and theirassociated diffraction collimators.

This resultant configuration can be considered as a series of concentricbut “independent diffraction systems” i.e. each conical shell and itscorresponding collimator form a discrete system as there is nominally nodiffracted flux coupling between each system.

The sizes of the inspection volume and depth or range resolution can bechosen with respect to the particular intended use of the system.

The diffracted flux collection heights can for example be arranged to beequidistant from one another to provide structural analysis at equalincremental heights throughout the working height e.g. securityscreening of electronic devices; distributed at different incrementaldistances to match better the density of sampling to any expectedheterogeneity in the sample e.g. collect diffracted flux from materialof interest above and or below a more strongly diffracting plane e.g.plastic explosive around a strongly diffracting copper ground plane in aprinted circuit board; or arranged to be coincident at one specifiedplane to increase the total amount of diffracted flux from a thinsample, which could then provide increased scan speed for manufacturingapplications and processes.

Similarly, the two-theta angles can be constant, or different two-thetaangles can be employed to provide increased d-spacing range for example,relatively large two-theta for the softer part of the spectrum for smalld-spacing analysis; and smaller two-theta for harder radiation and bigd-spacing analysis.

As a further variation, one could employ more than one diffractioncollimator at a different two-theta angle but different heights perconical shell beam to increase the total amount of signal collected thusimproving speed and signal to noise ratio (and provide increased depthresolution).

It is also possible to employ more than one diffraction collimator atthe same two-theta angle but different heights per conical shell beam toincrease the total amount of signal collected thus improving speed andsignal to noise ratio (and provide increased depth resolution).

Furthermore, it is possible to employ diffraction collimators arrangedto collect diffracted flux between the primary beam and the symmetryaxis of the system as shown in FIG. 23 and/or arranged to collectdiffracted flux on the opposite side (i.e. mirror image) of the primarybeam in comparison to that shown in FIG. 23.

Various modifications and improvements can be made to the above withoutdeparting from the scope of the disclosure. It is to be appreciated thatwhile references to “a sample” have been made, the disclosure can beapplied to the inspection of many samples at the same time.

What is claimed is:
 1. A sample inspection apparatus comprising: asource of electromagnetic radiation; a beam forming collimator forproducing a substantially conical shell of radiation, a first radiationdetector arranged to receive diffracted radiation after incidence of theconical shell beam upon a sample to be inspected; a coded apertureprovided at a detection surface of the first radiation detector; and arange detector which is arranged to collect radiation transmittedthrough the coded aperture.
 2. The sample inspection apparatus of claim1, wherein the coded aperture is provided at a point where a radiationhotspot is expected to form.
 3. The sample inspection apparatus of claim1, wherein the coded aperture is provided at a position along a symmetryaxis of the apparatus.
 4. The sample inspection apparatus of claim 1,comprising a coded aperture mask providing a plurality of apertures, andcentral occluder provided at one or more of the apertures of the codedaperture mask to eliminate stray illumination.
 5. The sample inspectionapparatus of claim 1, comprising a system for storing and analysing datathat is collected by the first radiation detector and/or the rangedetector.
 6. The sample inspection apparatus of claim 1, wherein thecoded aperture and the first radiation detector are integrated.
 7. Thesample inspection apparatus of claim 1, wherein a body of an aperturecomprises angled sidewalls that taper towards a centre point.
 8. Amethod of inspecting a sample, comprising: providing a source ofelectromagnetic radiation; producing a substantially conical shell ofradiation using a beam forming collimator; providing a first radiationdetector arranged to receive diffracted radiation after incidence of theconical shell beam upon a sample to be inspected; providing a codedaperture provided at a detection surface of the first radiationdetector; and providing a range detector which is arranged to collectradiation transmitted through the coded aperture.